The present invention relates to a method for plugging a hole. The invention also relates to a cooling element.
In general, the invention relates to a method for plugging a hole, in which method, in a hole provided in an object essentially made of mainly copper, such as the housing element of a cooling element, there is inserted a plug mainly made of copper. Said holes to be plugged according to the object of the invention exist for example in cooling elements employed for cooling the structures of furnaces used in metal industry, such as flash smelting furnaces or blast furnaces used in steel industry, or in connection with the cooling channels of chutes used for conducting the molten metal. Typically the cooling elements are made of copper and provided with for example longitudinal and/or transversal channels where the cooling agent circulates. Part of the holes forming the cooling element channel system are plugged, so that in the element, there is left only a necessary amount of inlet holes through which the cooling agent is conducted into the element, and outlet holes through which the cooling agent is conducted out of said element. According to a known plugging method, the cooling element is provided with a plug that is arranged in the hole to be plugged by a pressed joint which is externally welded to the cooling element housing, typically with a weld that extends to the depth of about 6 mm. Prior to welding, the working pieces are preheated up to a high temperature. In the preheating step, the oxidizing risk of the plug joint is high, and at that point the prior art plugging joint is relatively vulnerable to damages caused by corrosion, among others. For instance the atmosphere prevailing in the furnace space of a flash smelting furnace and containing, among others, SO2 gas, causes corrosion that proceeds as a sulphatizing reaction. In order to avoid the risk of damaging the plugging joint and possible leaking of the cooling agent as a result, it has been necessary to replace the cooling elements fairly often.
The object of the present invention is to realize a new method for plugging a hole, particularly the hole of a cooling element, whereby the drawbacks of the prior art are avoided. Thus another object of the invention is to realize a novel cooling element that has a longer working life than the prior art elements.
In the method according to the invention, between the plug side surface and the hole inner surface, there is arranged brazing agent with a melting temperature lower than the melting temperature of the parts to be joined together, and at least the junction area of the plug and the housing element of the object, such as the housing element of a cooling element, is heated at least up to the mewing temperature of the brazing agent or to the vicinity thereof, whereafter the juncture area is cooled. By means of the method according to the invention, there is achieved a joint that extends to a desired depth in the longitudinal direction of the plug. According to a preferred embodiment of the invention, the brazing agent is in the form of foil. In that case the brazing agent is in a form that is easy to handle, and it can be placed in the juncture exactly at the desired spot, so that the making of joints is carried out without difficulty.
According to an embodiment of the method according to the invention, the brazing agent is chosen from a group including as compounds Ag+Cu, Al+Cu, Sn+Cu and Sb+Cu. In the method according to the invention, the joint is made by applying a diffusion technique, where the components of copper or copper alloy and the brazing agent are partly dissolved on the contact surface. In this way, there is obtained an extremely high-quality joint. In the case of copper alloys, this is made possible by compositions that occur at least in the alloy systems Cuxe2x88x92Ag, Cuxe2x88x92Al, Cuxe2x88x92Sn and Cuxe2x88x92Sb, which melt at relatively low temperatures and are rich or even pure as regards the alloy components. Thus, in some embodiments it is possible to use for instance an exclusively Al-bearing foil. Moreover, the phase balances of said binary alloys can in the method be adjusted by moving over to ternary alloy balances (or balances of even more components). According to a preferred embodiment of the invention, the brazing agent has an eutectic composition with a given alloy composition with copper. By applying the brazing agents required for the diffusion joint in sufficiently thin, optimized layers, with diffusion mechanisms it is possible, even at low working temperatures, to obtain in the final structure solid phases that melt at a higher temperature, even by hundreds of degrees, than the original brazing agent. Thus the joint becomes, in a way, self-repairing, because it endures higher temperatures than the pure brazing agent would endure. The joining can be carried out for example by means of foil (brazing strip), or brazing wire; or else on the surfaces to be joined together, there can be used thin alloy material layers that are provided on said surfaces in advance. The heat required by the diffusion mechanisms between the brazing agent and the pieces to be joined together can be produced by heating with a heating means, for instance a liquefied gas burner. Other suitable heating arrangements for the heating of the juncture area can also be used, for example induction heating. If, in addition to the diffusion joint, in the junction there also is used surface welding of the plug, the welding temperature of the surface welding can be likewise be utilized when making the diffusion joint.
On the surface of the brazing foil and/or at least on one of the surfaces to be joined together, there is brought a layer of tin (Sn) prior to making the joint. The application of tin lowers the temperature required for making the joint. Moreover, the oxidation of the surfaces to be joined is avoided, and shielding gas arrangements are not needed in connection with making the joint. In order to start the phase transition reactions and to obtain an optimal seam structure, there suffices a tin layer of the order of a few micrometers between the Ag+Cu foil and the piece to be joined. The technique according to the method is not critical for the Ag+Cu composition, wherefore the use of an essentially pure Ag foil also is possible. The creation of the joint takes place rapidly, as a result of a diffusion of molten and solid materials as well as the successive phase transition reactions, already at the preheating temperatures prepared for the welding. By means of an application of the method according to the invention, it has been possible to create brazing agents that are well resistant to an atmosphere containing the gases present in the furnace space. For instance a brazing agent Ag+Cu endures well the SO2 bearing atmosphere of a flash smelting furnace. With respect to corrosion taking place in the working conditions, the tin of the created mosaic gold is not harmful, because it is not sulphatized in similar fashion as zinc and copper. As for the silver that is dissolved in the phases of the junction seam, it improves the resistance to corrosion of the mosaic gold.
The invention also relates to a cooling element for furnaces, said element comprising a housing element mainly made of copper, and channels formed in said housing element for the cooling agent circulation; when forming said channels, in the housing element there are made holes that open on the surface thereof, and at least part of said holes are plugged. The invention is characterized in that the joint between the plug and the housing element is a diffusion joint. The cooling element according to the invention is extremely well resistant to the atmosphere prevailing in furnaces. The joint surfaces are mainly formed of the plug side surface and the hole inner surface. Thus the joint is made sufficiently long in the longitudinal direction of the plug. The joint is realized by using brazing foil arranged in between the joint surfaces. The plug comprises a threaded section and a conical joint section. By means of the threaded section, the joint surfaces are pressed together, so that a high-quality joint is formed. The joint section of the plug is made self-centering, so that the joint surfaces are set evenly against each other.